visible to the others. The powers of the stations in this respect are exceedingly various. The station on Putney Heath, communicating with Chelsea, is generally rendered useless during easterly winds by the smoke of London, which fills the valley of the Thames between this spot and Chelsea hospital; or more commonly between the shorter distance of the Admirality and Chelsea. Dead flats are found to be universally unfavourable; and generally stations are useless nearly in the proportion of the miles of dead flat looked over. On the contrary, stations between hill and hill, looking across a valley, or series of valleys, are mostly clear; and water surfaces are found to produce fewer obscure days than land in any situation. The period least favourable of the same day is an hour or two before and after the sun's passage of the meridian, particularly on dead levels, where the play of the sun's rays on the rising exhalations renders distant vision exceedingly obscure. The tranquility of the morning and evening are ascertained to be the most favourable hours for observation.

A message from London to Portsmouth, is usually transmitted in about fifteen minutes; but, by an experiment tried for the purpose, a single signal has been transmitted to Plymouth and back again in three minutes, which, by the Telegraph route, is at least five hundred miles. In this instance, however, notice had been given to make ready, and every captain was at his post to receive and return the signals. The progress was at the rate of one hundred and seventy miles in a minute, or three miles per second, or three seconds at each station; a rapidity truly wonderful! The English telegraph consists of a large frame, in which are placed and worked six shutters, marked in the plate a, b, c, d, e, f, by means of ropes pulled in the manner of bell ropes. The number of signals produced by it is sixty-three-by which are represented the ten digits, the letters of the alphabet, many generic words, and all the numbers which can be expressed by sixty-three variations of the digits. The signals are sufficiently various to express any three or four words in twice as many changes of the shutters.

The observers at these telegraphs are not expected to keep their eye constantly at the glass, but look only every five minutes for the signal to make ready. The telescopes

are Dolland's Achromatics, which possess no recommendation but their enlarged field, and their freedom from prismatic colours in that field; points of no consequence in looking through a fixed glass at a fixed and circumscribed object. The field of the Galilean telescope is quite large enough, and having, instead of the six contained in Dollond's achromatics, but two lenses, one of which is a thin concave, it exhibits the object with greater brightness, and therefore ought to have been preferred for this purpose. It seems strange also, that, to ease the operator, it has never been contrived to exhibit the fixed spectrum, on the principle of a portable camera, so that, without wearying the eye, the changes of the distant telegraph might have been exhibited on a plane surface, and seen with both eyes.

THE AIR BALLOON.

[See Plate, No. 82.]

AMONG the many discoveries of modern philosophy this is one of the most splendid: hitherto, however, it has not been attended by corresponding utility, owing to the difficulty of steering the machine. The most promising attempts to overcome this difficulty were made by Zembeccari, an Italian, whose aerial excursions are curiously detailed by Kotzebue, in his travels, and whose principles were truly scientific; but still this great desideratum remains to be attained.

The discovery of hydrogen gas, which is 15 times lighter than atmospheric air, suggested the plan of filling with this gaseous substance a silken balloon, and of its ascent in the air, with an aeronaut appended to it, provided the whole should not exceed the weight of an equal bulk of atmospheric air. The process of filling the balloon is accomplished by mixing five parts of water with one of sulphuric acid, and pouring the mixture on iron filings: the light gas, by the decomposition of the water, will rise into the balloon; and the balloon, being 12 times lighter than the atmospheric air, will rise through it. Thus have two, three, and even four persons, been at one time carried through the atmosphere.

More than fifty aerial voyages, in different parts of Europe, have been made by Blanchard; nearly as many by Garnerin; and thirty by Mr. Sadler.

THE STEAM ENGINE.

[See Plate, No. 83.]

THIS engine consists of a large cylinder or barrel, in which is fitted a solid piston like that of the forcing pump. Steam is thus supplied from a large boiler, which in forcing up the piston, instantly opens a valve, through which cold water rushes on the principle of the common pump. Other steam is then introduced, which forces it down again, and drives the water out of the pipe with immense force. The steam then raises the piston again, and again makes it fall by which alternate motion the grandest operations are performed. The action of the piston moves up and down a large beam; and this beam communicates to other machinery the power of 100 or 200 horses.

The power of some of the steam engines constructed by Messrs. Boulton and Watt, is thus described, as taken by actual experiment. An engine, having a cylinder of 31 inches in diameter, and making 17 double strokes per minute, performs the work of 40 horses, working night and day, (for which three relays, or 120 horses. must be kept) and burns 11,000 pounds of Staffordshire coal per day. A cylinder of 19 inches, making 25 strokes of 4 feet each per minute, performs the work of 12 horses, constantly lalabouring, and burns 5,700 pounds of coals per day.These engines will raise more than 20,000 cubic feet of water, 24 feet high, for every hundred weight of good pit coal consumed by them.

The principal of Watt's improved engine, represented in the cut, is the same as the above, but the economy is still greater. The steam which is below the piston escapes into the condenser A, by the cock B, which is opened by the rod C, and at the same time the steam is admitted by the cock D into the upper part of the cylinder: when the piston has descended, the cocks E and F act in a similar manner in letting out the steam from above, and admitting it below the piston. The jet is supplied by the water of the cistern G, which is pumped up at H, from a reservoir: it is drawn out, together with the air which is extricated from it, by the air pump I, which throws it into the cistern K, whence the pump L raises it to the cistern M, and it enters

the boiler through a valve which opens whenever the float W descends below its proper place. The pipes O and P serve also to ascertain the quantity of water in the boiler. The piston rod is confined to a motion nearly rectilinear, by the frame Q. The fly-wheel R is turned by the sun and planet wheel S T, and the strap U turns the centrifugal regulator W, which governs the supply of steam by the valve or stop cock X.

STEAM BOATS.

[See Plate, No. 86.]

THE description of the Clyde steam boat, represented in the plate, is as follows. Its extreme length is 75 feet, its breadth 14, and the height of the cabins 64 feet. She is built very flat, and draws from 2 feet 9 inches to 3 feet water. The best or after-cabin, is 20 feet long, and is entered from the stern: between the after-cabin and the engine a space of 15 feet is allotted for goods. The engine is a 12 horse power, and occupies 15 feet: the fore-cabin is 16 feet long, and is entered from the side. The paddles 16 in number, form two wheels of 9 feet diameter, and 4 feet broad, made of hammered iron: they dip into the water from 1 foot 3 to 1 foot six inches. Along the outer edge of these wheels a platform and rail are formed quite round the vessel, projecting over the sides, and supported by timbers reaching down to the vessel's side. This steam boat runs at the rate of 4 or 44 miles per hour in calm weather; but against a considerable breeze 3 miles only. It can accommodate 250 passengers, and is wrought by five men. The engine consumes 12 cwt. of coals per day. The funnel of the boiler is 25 feet high; and carries a square sail 22 feet in breadth.

To convey a precise idea of the utility of steam boats, and to quiet the apprehensions entertained relative to their safety, the following details, by Sir Richard Phillips, have appeared in the Monthly Magazine.

The groundless alarmis relative to a supposed increase of danger from travelling by Steam-packets, led the editor of the Monthly Magazine, within the current month, (July, 1817) to make a voyage in one of them from London to Margate. This vessel left her moorings, at the Tower of London, about half past eight in the morning, at the time

the tide was running strong up the river, and when no other vessel could make progress, except in the direction of the tides. The steam-packet proceeded, however, against the stream, in a gallant style, at the rate of six or seven miles an hour; and a band of music, playing lively airs on the deck, combined with the steadiness of the motion, to render the effect delightful. An examination of the steam engine, and of her rate of working, proved that no possibility of danger exists. It appeared that the boiler had been proved at twenty-five pounds to the square inch; but that the valve was held down by a weight of only four pounds, and that the mercurial gauge did not indicate an employment of actual pressure of above two pounds and a hair per square inch. Hence it follows, that, although the engine was capable of sustaining a pressure of at least twenty-five pounds, only four pounds, or less than a sixth, was the whole force which the valve would permit to be exerted; and that, in point of fact, a pressure of only two pounds and a half to the square inch, or only one tenth of the proven power of the boiler, was employed. There is, therefore, less danger in passing some hours in contact with such a machine, than there is in sitting near a boiling teakettle, tea-uru, or saucepan, under circumstances in which they are often used. Opposite Greenwich a fine commentary was afforded of the value of steam as a navigating power, in preference to winds and tides, a Margate sailingpacket passing towards London, which had been a day and two nights on its passage, a period of time which it appears is not uncommon. In short, with uninterrupted pleasure, and in an hour sooner than the captain had named at starting, the vessel was carried along-side Margate-pier, having employed nine hours in performing a voyage of ninety miles. In this case it appeared, that a pressure of two pounds to the square inch produced about forty rotations per minute of the acting water wheels; and, as these were ten feet in diameter, the motion of the impelling floats, or wheel-paddles, would be at the rate of fifteen with or against the stream, at an average of ten miles an hour. The consumption of coals during the voyage was less than a chaldron; but it was described as amounting frequently to a chaldron and a half On the whole, nothing could be more demonstrative of the worth and security of